A Diagnostic Technique for Particle Characterization Using Laser Light Extinction

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Abstract

Increased operations of aircraft, both commercial and military, in hostile desert environments have increased risks of micro-sized particle ingestion into engines. The probability of increased sand and dust ingestion results in increased life cycle costs, in addition to increased potential for performance loss. Thus, abilities to accurately characterize inlet sand would be useful for engine diagnostics and prognostic evaluation. Previous characterization studies were based on particle measurements performed a posteriori. Thus, there exists a need for in situ quantification of ingested particles.
The work presented in this thesis describes initial developments of a line-of-sight optical technique to characterize ingested particles at concentrations similar to those experienced by aircraft in brownout conditions using light extinction with the end goal of producing an onboard aircraft diagnostic sensor. By measuring the extinct light intensity in presence of particles over range of concentrations, a relationship between diameters, concentration and light extinction was used for characterization. The particle size distribution was assumed log-normal and size range of interest 1-10 μm.
To validate the technique, particle characterization in both static and flow based tests were performed on polystyrene latex spheres of sizes 1.32 μm, 3.9 μm, 5.1 μm, and 7 μm in mono-disperse and poly-disperse mixtures. Results from the static experiments were obtained with a maximum relative error of 11%. Concentrations from the static experiments were obtained with a maximum relative error of 18%. Mono-dispersed and poly-dispersed particle samples were sized in a flow setup, with a maximum relative error of 12% and 10% respectively across all diameter samples tested. Uncertainty in measurements were quantified, with results indicating a maximum error of 17% in diameters due to sources of variability and showed that shorter wavelength lasers provide lower errors in concentration measurements, compared to longer wavelengths.
For real time, on-board measurements, where path lengths traveled by light are much larger than distances traveled in initial proof of concept experimental setups, requirements would be to install sensitive detectors and powerful lasers to prevent operation near noise floors of detectors. Vibration effects from the engine can be mitigated by using larger area collection optics to ensure that the transmitted light falls on active detector areas.
Results shown in this thesis point towards validity of the light extinction technique to provide real time characterization of ingested particles, and will serve as an impetus to carry out further research using this technique to characterize particles entering aircraft engine inlets.